1. Field of the Invention
This invention relates to duplex radio telephone systems and, more specifically, to cordless telephone systems.
2. Description of the Prior Art
Present day cordless telephone systems fall, generally, into two categories. The first of those categories is the cordless system which operates totally within the Citizens' Band spectrum, which is from 26.965 Mhz to 27.255 Mhz. In such systems the base unit (which is connected to the fixed telephone line through a coupler or directly) transmits, on a first frequency in that band of frequencies, incoming telephone signals, such as the ringing signal and voice signal or other intelligence signals, from the fixed location to the remote location where the mobile unit which is in the form of a telephone but includes radio circuits, receives and operates on the signals from the base station. The mobile unit transmits, on a second frequency, or channel, in the Citizens' Band, the voice or other intelligence and control signals developed at the mobile unit. The operating channels of the base station and mobile unit are spaced in frequency as far as is practicable so as to permit duplex operation i.e., simultaneous transmission and reception on both ends of the system--the base station and the mobile unit.
In the second category of cordless phone systems, the base station transmits its control and information signals on a low frequency, e.g., at 1.7 Mhz and the mobile unit transmits its signals in the 26.965 to 27.255 Mhz Citizens' Band. With the wide spacing of channels used in the system no echo or sidetone is evident in the telephone receiver of the mobile unit when signals are being transmitted from the mobile unit.
An example of a cordless telephone system is set forth in U.S. Pat. No. 3,449,750 (Sweigert). The system in the Sweigert patent uses channels separated about 6 Mhz which is broad spacing with respect to the first category systems and narrow spacing with respect to the second category of systems discussed hereinbefore.
The second category of systems has the inherent problem that a resonant fraction of a wavelength is very long at the 1.7 Mhz frequency and an antenna for transmission or reception at that frequency must be physically long to be resonant. In view of that circumstance, it is conventional to rely upon the A.C. wiring from which the power for the base station is derived to act as the signal carrier and radiator for the base station in the 1.7 Mhz range of frequencies. Unfortunately, because of the very large capacity to ground experienced in the electrical conduit through which the power lines run, the R-F signal from the base station is repidly attenuated.
The effective length of radiator is only slightly longer than the power cord running from the base station to the A.C. wall socket. Such a short radiator results in a severly limited range for the mobile station because the mobile unit quckly loses the signal from the base station as the mobile unit is separated from the base station. Further, with prior art systems there was no positive indication at the remote or mobile unit when that unit moved out of the range for effective communication between it and the base station. Thus, the user of the mobile unit could be talking as he moved and be completely lost at the base station with the result that the user of the mobile station would be talking to no one and, when he discovered that fact, could be highly irritated.
Thus, it is an object of this invention to overcome in a cordless telephone system the problems and disadvantages set forth hereinbefore.
It is a further object of this invention to provide a cordless telephone system which will have an extended range for its low-frequency radio link.
It is a still further object of this invention to provide an improved cordless telephone system in which the user of the mobile unit is advised immediately upon his being separated from the base station a distance greater than the operating range of the system.